As vapor deposition technique using plasma, vapor deposition technique using vapor phase plasma has conventionally been used widely. However, in vapor phase plasma, which has a low material density, the reaction rate is low. Therefore, there is a need for technology for generating plasma in a liquid, which has a high material density.
For example, the pamphlet of International Publication No. 02/038827 (Document 1) describes an invention of producing a stream of bubbles containing a source of a material to be deposited in an electrolyte in which a pair of spaced electrodes are disposed and generating plasma glow discharge in the bubble region, thereby depositing the material on one of the electrodes.
However, the invention recited in Document 1 makes use of direct current glow discharge. Document 1 partially mentions assisting glow discharge generation by microwave or electromagnetic wave radiation, but gives no specific description about it. There are many unclear points from a technical viewpoint and the technique recited in Document 1 is considered to be nothing but direct current glow discharge. Therefore, the reaction rate is assumed to be at the same level as vapor phase plasma.
Moreover, the bulletin of Japanese Unexamined Patent Publication No. 2002-301136 (Document 2) also describes irradiating a liquid with microwaves from outside a vessel and thereby decomposing hazardous substances in the liquid. Document 2 is to radiate microwaves from outside a vessel and thereby decompose hazardous substances contained in a liquid in the vessel, but does not explain by what mechanism the hazardous substances are decomposed. Therefore, it is unlikely that this manner of microwave radiation generates plasma in the liquid, and Document 2 does not describe that plasma was generated in the liquid. Even if plasma generation in the liquid is not impossible, there will be a need to supply extremely large electric power and it is of little practical use.
In addition, in the bulletin of Japanese Unexamined Patent Publication No. 2003-297598 (Document 3) and the bulletin of Japanese Unexamined Patent Publication No. 2004-152523 (Document 4), the inside of a liquid such as dodecane is irradiated with ultrasonic waves by an ultrasonic wave generator, thereby generating bubbles in the liquid, and at the same time the liquid at a position where the bubbles are generated is irradiated with electromagnetic waves by an electromagnetic wave generator, thereby generating high energy plasma in the bubbles.